Turbomachine casing assembly

ABSTRACT

A turbomachine casing assembly includes a first casing element locatable radially outward of one or more rotating aerofoil elements of a turbomachine and a second casing element located radially distal to the first casing element. A void is provided between a radially proximal face of the first casing element and a radially proximal face of the second casing element, at the first end of the first casing element. The first end of the first casing element is located against a corresponding first end of the second casing element by an energy absorbing element, with the first casing element; including a cantilever. Release of one of the rotating aerofoil elements results in the element, or part thereof, impacting the first casing element which results in the cantilever bending into the void provided between the first and second casing elements.

This invention claims the benefit of UK Patent Application No.1201176.3, filed on 25 Jan. 2012, which is hereby incorporated herein inits entirety.

FIELD OF THE INVENTION

This invention relates to a turbomachine casing assembly andparticularly, but not exclusively, to a casing assembly for the fan of aturbofan gas turbine engine.

BACKGROUND

Turbofan gas turbine engines for powering aircraft generally compriseinter alia a core engine, which drives a fan. The fan comprises a numberof radially extending fan blades mounted on a fan rotor which isenclosed by a generally cylindrical fan casing.

To satisfy regulatory requirements, such engines are required todemonstrate that if part or all of a fan blade were to become detachedfrom the remainder of the fan, that the detached parts are suitablycaptured within the engine containment system.

It is known to provide the fan casing with a fan track liner whichtogether incorporate a containment system, designed to contain anyreleased blades or associated debris. FIG. 1 shows a partialcross-section of such a casing and fan track liner.

In the event of a “fan blade off” (FBO) event, the detached fan blade 8travels radially outward and forwards. In doing so, it penetrates theattrition liner 10. It may also penetrate the septum 12 and aluminiumhoneycomb layer 14 before engaging the hook 18. The fan track liner musttherefore be relatively weak in order that any released blade orfragment thereof can pass through it essentially unimpeded andsubsequently be trapped by the fan casing.

In addition to providing a blade containment system, the fan track linerincludes an annular layer of abradable material which surrounds the fanblades. During operation of the engine, the fan blades rotate freelywithin the fan track liner. At their maximum extension of movementand/or creep, or during an extreme event, the blades may cut a path intothis abradable layer creating a seal against the fan casing andminimising air leakage around the blade tips.

The fan track liner must also be resistant to ice impact loads. Arearward portion of the fan track liner is conventionally provided withan annular ice impact panel. This is typically a glass-reinforcedplastic (GRP) moulding which may also be wrapped with GRP to increaseits impact strength. Ice which forms on the fan blades is acted on byboth centrifugal and airflow forces, which respectively cause it to moveoutwards and rearwards before being shed from the blades.

The geometry of a conventional fan blade is such that the ice is shedfrom the trailing edge of the blade, strikes the ice impact panel and isdeflected without damaging the panel.

Swept fan blades are increasingly used in turbofan engines as they offersignificant advantages in efficiency over conventional fan blades. Sweptfan blades have a greater chord length at their central portion thanconventional fan blades. This greater chordal length means that ice thatforms on a swept fan blade follows the same rearward and outward path ason a conventional fan blade but may reach the radially outer tip of theblade before it reaches the trailing edge. It will therefore be shedfrom the blade tip and may strike the fan track liner forward of the iceimpact panel within the blade off zone.

The liner used with a swept fan blade is therefore required to be strongenough to resist ice impact whilst allowing a detached fan blade topenetrate and be contained therewithin.

In recent years there has been a trend towards the use of lighter fanblades, which are typically either of hollow metal or of compositeconstruction. These lighter blades have a similar impact energy per unitarea as an ice sheet, which makes it more difficult to devise a casingarrangement that will resist the passage of ice and yet not interferewith the trajectory of a released fan blade.

It is an objective of this invention to provide a gas turbine enginecontainment assembly that will substantially overcome the problemsdescribed above, and that is particularly suited for use with composite,hollow metallic, or other lightweight, fan blades.

STATEMENTS OF INVENTION

According to an aspect of the present invention there is provided aturbomachine casing assembly, comprising:

-   -   a first casing element locatable radially outward of one or more        rotating aerofoil elements of a turbomachine;    -   a second casing element located radially outward of the first        casing element; and    -   a void, provided between the first casing element and the second        casing element, at a first end of the first casing element;    -   wherein the first end of the first casing element is located        against a corresponding first end of the second casing element        by at least one pocket disposed at the first end of the first        casing element, and    -   the first casing element comprises a cantilever, the cantilever        being arranged in a region extending along the first casing        element from the first end and having a pivot axis at a mid        portion of the first casing element;    -   whereby upon release of one of the rotating aerofoil elements,        the cantilever bends into the void provided between the first        and second casing elements, which causes the at least one pocket        to deform and to thereby absorb at least some of the impact        energy resulting from the released rotating aerofoil element.

The casing assembly of the present invention allows a shed fan blade topenetrate the first casing element at a first, or forward, end, whilstremaining rigid to ice impact at the opposite, or rear, end. Thecompeting requirements of fan blade ice shedding loads and fan blade offloads may thus be accommodated in a way that was not previouslypossible. The manner in which this is done allows for the potential totune the casing assembly to correctly service each requirement and doesso whilst saving weight and easing manufacture.

In addition, by forming a void within the casing assembly, the weight ofthe assembly can be reduced. This increases the efficiency of the engineand thus makes it more attractive for aircraft applications.

A benefit of the energy absorbing element being formed within the firstcasing element is that no additional components are required for thecomplete casing assembly. This makes the assembly simple and convenientto install.

Optionally, the or each pocket is formed from a sheet material selectedfrom the group comprising thermoplastics, carbon fibre reinforcedplastic, glass fibre reinforced plastic, aramid fibre reinforcedplastic, boron fibre reinforced plastic and composites thereof.

Since the casing assembly is intended for use in aircraft applications,it will be subjected to significant variations in atmospheric pressureas a result of changes in altitude. It is therefore necessary for thepocket to be dimensionally stable when subjected to a range of externalatmospheric pressures. The use of carbon fibre, glass fibre or othercomposite material can assist in maintaining the dimensional integrityof the pocket.

Optionally, the or each pocket comprises a skin formed from a rigidmaterial and the or each pocket contains a fluid.

Optionally, the or each pocket is formed from a foam material.

Optionally, each of the one or more rotating aerofoil elements comprisesa first cutting portion on a radial end thereof.

The use of a first cutting portion on a radial end of the aerofoilelement ensures that the pocket is reliably penetrated and collapsedduring a FBO event. Once the pocket has collapsed, the hook is exposedin readiness for the blade being captured.

Optionally, a second cutting portion is disposed at the first end of thefirst casing element.

Optionally, a force transfer feature is disposed within the or eachpocket, the force transfer feature extending between the proximal faceof the first casing element and the second cutting portion.

The force transfer feature provides a direct load path between theproximal face of the first casing element and the second cutting portiondisposed at the first end of the first casing element. This results inthe impact of a detached blade being more likely to initiate the ruptureof the or each pocket.

Optionally, the or each pocket contains a liquid.

The use of a liquid as a filler for the pocket has the benefit of makingthe pocket less susceptible to changes in dimensional form with changesin altitude (i.e. variation in atmospheric pressure).

Optionally, the liquid is a contact adhesive which catalyses when incontact with air.

After a detached blade has penetrated the pocket, it is desirable forthe pocket to remain in a collapsed condition in order to ensure thatthe detached blade and any debris remains trapped within the casingassembly. This may be achieved by adding to the pocket a small amount ofa contact adhesive which cures upon contact with air which ensures thatthe pocket remains in a collapsed condition. Adhesive that escapes thepocket may advantageously trap stray debris which may be useful inpost-FBO event analysis.

Other aspects of the invention provide devices, methods and systemswhich include and/or implement some or all of the actions describedherein. The illustrative aspects of the invention are designed to solveone or more of the problems herein described and/or one or more otherproblems not discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

There now follows a description of preferred embodiments of theinvention, by way of non-limiting example, with reference being made tothe accompanying drawings in which:

FIG. 1 shows a partial, sectional view of a conventional fan track lineras used in a gas turbine engine casing;

FIG. 2 shows a partial, sectional view of a fan casing assembly, notforming part of the present invention;

FIG. 3 shows a partial, sectional view of a fan casing assembly, notforming part of the present invention;

FIGS. 4 a and 4 b show details of alternative fastening arrangements forthe fan casing assembly of FIG. 3;

FIG. 5 shows a perspective view of a fan track liner, not forming partof the present invention;

FIG. 6 shows a partial, perspective view of a fan track liner, notforming part of the present invention;

FIG. 7 shows a perspective view of a fan track liner, not forming partof the present invention;

FIG. 8 shows a partial, sectional view of a fan casing assemblyaccording to a first embodiment of the invention;

FIG. 9 shows a partial, sectional view of a fan casing assemblyaccording to a second embodiment of the invention;

FIG. 10 shows a partial, sectional view of an alternative arrangement ofthe fan casing assembly of FIG. 8;

FIG. 11 shows a partial, sectional view of the fan casing assembly ofFIG. 8 in which the pocket incorporates one or more cutting features;

FIG. 12 shows a partial, sectional view of the fan casing assembly ofFIG. 8 including a force transfer feature incorporating a cuttingfeature;

FIG. 13 shows a partial, sectional view of a fan casing assembly, notforming part of the present invention; and

FIGS. 14 a and 14 b show partial, sectional views of the fan casingassembly of FIG. 13, before and after the impact of a fan blade.

It is noted that the drawings may not be to scale. The drawings areintended to depict only typical aspects of the invention, and thereforeshould not be considered as limiting the scope of the invention. In thedrawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION

Referring to FIG. 2, a fan casing assembly, not forming part of thepresent invention, is designated generally by the reference numeral 100and comprises a first casing element 110 and a second casing element120.

The first casing element 110 has a first end 112, a radially proximalsurface 114 and a radially distal surface 116, and the second casingelement 120 has a first end 122, a radially proximal surface 124 and aradially distal surface 126.

The first casing element 110 at least partially encloses one or morerotating aerofoil structures 130. These aerofoil structures 130 maycomprise blades of a turbomachine, in particular compressor fan blades.The second casing element 120 is disposed radially distal to the firstcasing element 110.

The turbomachine casing assembly 100 comprises a plurality of firstcasing elements 110 circumferentially disposed about a curve defined bythe blade tip path of the one or more aerofoil structures 130 of theturbomachine. The first and/or second casing elements 110,120 maytypically be metallic and may, for example, be formed of aluminium,titanium, steel or any other suitable metal. The first and/or secondcasing elements 110,120 may typically be formed from fibre reinforcedcomposite materials which may include some integral metallic features.

The first end 112 of the first casing element 110 is aligned with thefirst end 122 of the second casing element 120 and is maintained in itsradial position relative to the fan blades 130 by an energy absorbingelement in the form of a crushable collar 140.

In an alternative arrangement, the energy absorbing element takes theform of a fusible or stretchable fastener 142, such as a bolt, as shownin FIG. 3. The stretchable bolt 142 connects the first end 112 of thefirst casing element 110 to the first end 122 of the second casingelement 120. As shown in FIG. 5, these fasteners 142 extend throughholes 144 in a forward rail 118 extending across the width of the firstend 112 of the first casing element 110. As a further alternative, theseholes 144 further comprise slots 146 a,146 b, as shown in FIGS. 4 a and4 b. The use of slotted holes allows the first end 112 of the firstcasing element 110 to hinge and move axially whilst the fasteners 142stretch to failure.

A ratchet retention mechanism 160 is provided at the juncture of thefirst ends 112,122 of the first and second casing elements 110,120. Thismechanism 160 takes the form of a linear rack 162 which is disposed atthe first end 122 of the second casing element 120 and a pawl 164disposed on the first end 112 of the first casing element 110. The pawl164 engages with the rack 162 such that the first casing element 110 isable to move radially outwards but is not able to return to its originalposition.

The casing assembly 100 comprises an infill member 170, in the form of astructural liner positioned between the proximal 114,116 and distalsurfaces of the first casing element 110. The infill member 170 may beformed from a frangible or crushable structure, such as acoustic foam orhoneycomb material.

The honeycomb material may be formed from a metal, such as aluminium, orfrom a non-metallic material, such as Nomex™ (a flame resistant aramidmaterial).

In an alternative embodiment of the invention, the infill member 170 maybe formed as a separate, discrete layer positioned between the first andsecond casing elements 110,120.

Each first casing element 110 includes, on its radially proximal surface114, an abradable layer 117 (see FIG. 5). The abradable layer 117 isattached to the infill member 170. An exemplary material for theabradable layer 117 is an epoxy resin or a polysulphide sealant, whichmay be curable at room temperature. The abradable layer 117 provides asurface against which the fan blade 130 is able to rub and cut a pathfor itself. For example, the fan blades 130 may rub against theabradable layer 117 and form a seal during normal engine operation.

Optionally, a septum layer (not shown) may be provided as an interlayerbetween the abradable layer 117 and the infill member 170. The septumlayer may be metallic or may be formed from a carbon fibre and/or glassfibre reinforced composite material.

As shown in FIG. 2, the forward portion of the distal face 116 of thefirst casing element 110 is angled relative to the correspondingproximal face 124 of the second casing element 120 such that a void 134is defined between the first and second casing elements 110,120 at theirforward, or first, ends 112,122.

The relative angled arrangement between the forward portions of thefirst and second casing elements 110,120 results in the forward portionof the first casing element 110 forming a cantilever structure 180. Thecantilever 180 is arranged in the region extending along the firstcasing element 110 from the first end 112 and having a pivot axis 182 ata mid portion of the first casing element 110.

The cantilever 180 is arranged such that upon release of one of the fanblades 130, the cantilever 180 bends into the void 134 provided betweenthe first and second casing elements 110, 120. The length of thecantilever 180 is arranged to be large enough to provide purchase forthe fan blade 130 and/or to develop enough bending moment to causecollapse of the first casing element 110 at the cantilever pivot 182.

The distal (radially outer) surface 116 of the first casing element 110may be provided with a linear array of rhomboid apertures 184 (see FIG.5) which are formed across the width of the first casing element 110 atthe cantilever pivot axis 182. In addition, the infill member 170 issevered at the pivot axis 182.

In other arrangements, the first casing element 110 may be provided withapertures 184 of an alternative shape. Alternatively, in preference toproviding apertures 184 on the distal surface 116, the ply orientationof the structure of the distal surface 116 may be locally modified atthe pivot axis 182.

A further alternative arrangement, shown in FIG. 6, involves cuttingthrough the distal surface 116 of the first casing element 110 at thecantilever pivot axis 182 and attaching a strong, flexible material,such as a Kevlar™ patch 186, across the pivot axis 182, to form a hinge.

A still further alternative, shown in FIG. 7, involves cutting throughthe distal surface 116 of the first casing element 110 and the infillmember 170 at the cantilever pivot axis 182 to form a slit 188, andcutting the distal surface 116 to form a plurality of finger portions192 which extend along the distal surface 116 of the first casingelement 110, with a plurality of ligaments 194 therebetween. Thesefinger portions 192 are attached to the distal surface 116 of the firstcasing element 110 with alternate finger portions 192 extending towardsopposite ends of the first casing element 110.

When the forward section of casing element 116 is forced outwards, theslit 188 widens and the hinge point 182 is allowed to fracture theligaments 194. This results in the separation of the first casingelement 110 into two sections with the forward section being allowed tomove forwards and radially outwards. The finger portions 192 serve toretain the two sections of the first casing element 110 together toprevent complete separation while allowing relative movement of theforward section.

In an alternative embodiment of the invention, the slit 188 may befilled with, for example, a non-setting filler or low tensile strengthfiller material. This would allow for the transmission of compressiveloads across the slit 188 whilst allowing the slit 188 to open when theforward section of the first casing element 110 hinges about the hingeportion 182.

The second casing element 120 comprises a fan blade retaining feature128 disposed at a forward, or first, end thereof, in the form of ahooked portion 128. The hooked portion 128 extends radially inwardly andthen axially in a rearward direction, and corresponds to the hook 18 ofFIG. 1.

In use, if a fan blade 130 becomes detached, it first strikes theproximal surface 114 of the forward portion of the first casing element110. The energy associated with this impact event is then transferred toand absorbed by the energy absorbing element, for example by thecompressive crushing of the crushable collar 140 and the infill member170. This allows the forward portion of the first casing element 110 todeform radially outwards, exposing the hook 128. As the detached blade130 continues to travel forwards and radially outwards, the blade tipengages with the hook 128 and subsequently becomes trapped in the casingassembly 100, thus preventing the blade 130 from travelling forward ofthe first end 122 of the second casing element 120.

Referring to FIGS. 8 and 9, a fan casing assembly according to sixth andseventh embodiments of the invention is designated generally by thereference numeral 200. Features of the fan casing assembly 200 whichcorrespond to those of fan casing assembly 100 have been givencorresponding reference numerals for ease of reference.

The fan casing assembly 200 has a first casing element 210 and a secondcasing element 120.

In this embodiment, the first casing element 210 has a first end 212, aradially proximal surface 214 and a radially distal surface 216. Theradially distal surface 216 of the first casing element 210 correspondsto the radially proximal surface 124 of the second casing element 120.

A rigid, hollow or fluid filled pocket 230 is disposed between theradially proximal and distal surfaces 214,216 at the first end 212 ofthe first casing element 210 and extends from the first end 212 towardsa mid portion of the first casing element 210. An infill member 270extends from the pocket 230 at the mid portion of the first casingelement 210 to the rear end thereof.

While the embodiment described shows a single pocket 230, in otherarrangements there may be two or more pockets.

The pocket 230 is filled with a pressurised gas. Alternatively, thepocket 230 may be filled with a liquid such as, for example, an epoxyresin. It may be advantageous for the epoxy resin to cure in response toUV radiation or to include a catalyst which cures in contact with air.In arrangements in which there is more than one pocket 230, a two partresin, or adhesive filler, may be utilised in which mixing of thecomponents takes place after the fan blade 130 has penetrated thepockets 230.

Alternatively, the pocket 230 may be formed as a region of foam material240, as shown in FIG. 9. In the arrangement of FIG. 9, the foam materialmay be provided on its radially innermost surface with an abradableskin.

In an alternative arrangement, shown in FIG. 10, a primary pocket 232 isdisposed at the first end 212 of the first casing element 210 and aplurality of secondary pockets 234 are disposed adjacent to the primarypocket 232 and extending behind a portion of the infill member 270towards the rear of the first case element 210. The primary andsecondary pockets 232,234 may or may not be in fluid communication withone another. This arrangement can provide a more progressive collapsemechanism to the impact by a fan blade 130.

The abradable layer 117 extends across both the infill member 270 andthe fluid-filled pocket 230 on the proximal surface of the first casingelement 210.

The pocket 230 itself is formed from a reinforced fabric material suchas, for example, carbon fibre or glass fibre reinforced compositematerials or a mixture thereof. Other materials, such as thermoplasticsand aramid or metallic fibres or foils, may be used to form the pocket.

A cutting portion 250 is provided on a forward portion of the fan blade130. This cutting portion 250 takes the form of a plurality of sharpprojections. The position of the cutting portion 250 on the blade tip isarranged such that during a heavy tip rub condition, the cutting portion250 does not contact the abradable layer 117.

In other arrangements, the cutting portion may comprise an abrasivecoating applied to a portion of the blade tip.

Alternatively, as shown in FIGS. 11 and 12, the cutting portion 260 maybe disposed on an inner surface of the first casing element 210. Inthese arrangements, the cutting portion 250 is disposed on an innersurface 218 of the first casing element 210 and is in direct contactwith the skin of the pocket 230. Consequently, impact energy applied tothe abradable layer 117 is transferred to the cutting portion 260 whereit may be applied to the pocket 230.

In the arrangement of FIG. 12, a force transfer feature 224 is providedwithin the pocket 230 which transfers a load applied to the proximalsurface 214 of the first casing element 210 to the interface between thecutting portion 260 and the pocket. In this way, no modification to thefan blade 130 to include a cutting portion 250 is required. As shown inFIG. 12, the force transfer feature 224 may be configured (in thisembodiment with holes/hollow sections) to allow deformation of the forcetransfer feature 224 so as not to overly impede the travel of thedetached blade 130 on its course to be trapped by the hook 128 and thesecond casing element 120.

In use, when the blade 130 becomes detached, it penetrates the abradablelayer 117 and the cutting portion 260 punctures the pocket 230. Thisallows the blade 130 to travel into the void defined by the pocket andexposes the hook 128 (see FIGS. 11 and 12) which engages with the blade130 to trap it within the casing assembly 200.

Referring to FIGS. 13 and 14, a fan casing assembly, not forming part ofthe present invention, is designated generally by the reference numeral300. Features of the fan casing assembly 300 which correspond to thoseof fan casing assembly 100 have been given corresponding referencenumerals for ease of reference.

The fan casing assembly 300 has a first casing element 310 and a secondcasing element 120.

In this embodiment, the first casing element 310 has a first end 312, aradially proximal surface 314 and a radially distal surface 316. Theradially distal surface 316 of the first casing element 310 abutsagainst the radially proximal surface 124 of the second casing element120.

The first casing element 310 includes, on its radially proximal surface,an abradable layer 117 which is attached to an infill member 370. Theabradable layer 117 has a proximal surface 350 and a distal surface 352.

The forward portion of the first casing element 310 comprises aplurality of resilient leaf elements 340 arranged in a stack and alignedwith the axis of the fan casing assembly 300.

In this embodiment, the length of each leaf element 340 increasesthrough the stack in a radially outward direction. Thus the shortestleaf element 340 is immediately adjacent to the abradable layer 117 andthe longest is furthest from the abradable layer 117.

The leaf elements 340 are formed from a resilient material such as, forexample a thermoplastic, carbon fibre or glass fibre laminate.

The leaf elements 340 are independent of one another and are mounted ina slotted elastomeric binder block 342 which is disposed laterallyacross the width of the first casing element 310. In this embodiment,the binder block 342 is formed from an elastomeric (rubber) material. Inother embodiments, the binder block 342 may be formed from analternative resilient material.

The binder block 342 abuts the forward edge of the infill member 370 andis angled rearwards. The leaf elements 340 are biased towards theproximal surface 314 of the first case element 310 and they may bethrough stitched 346 to maintain them in their biased position. In thisway, the biased stack of leaf elements 340 abuts the distal face 352 ofthe abradable layer 117.

The forward ends of the plurality of resilient leaf elements 340 extendbeyond the first end 312 of the first casing element 310 and arearranged to contact a distal surface of the hook 128.

In use, when a blade 130 becomes detached, it penetrates the abradablelayer 117 and impacts the stack of resilient leaf elements 340. Some ofthe energy of the impact is dissipated in tearing the stitching 346which holds the stack of leaf elements 340 together which results in theelements 346 deflecting and returning to their straight configuration.This in turn exposes the hook 128 which engages with the blade 130 totrap it within the casing assembly 300.

The casing assemblies disclosed herein are equally applicable to solidand hollow fan blades and may be used with light-weight (hollowline-core, or solid or hollow composite) fan blades. The casingassemblies may also be used with aerofoil structures, e.g. fan blades,comprising a principal load-carrying member at the front of the aerofoilstructure such as a picture frame or metallic sheath. The presentdisclosure may also be applied to swept or unswept aerofoil structures.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to aperson of skill in the art are included within the scope of theinvention as defined by the accompanying claims.

What is claimed is:
 1. A turbomachine casing assembly, comprising: afirst casing element located radially outward of one or more rotatingaerofoil elements of a turbomachine; a second casing element locatedradially outward of the first casing element; and a void, providedbetween the first casing element and the second casing element, at afirst end of the first casing element; wherein the first end of thefirst casing element is located against a corresponding first end of thesecond casing element by at least one pocket disposed at the first endof the first casing element, and the first casing element comprises acantilever, the cantilever being arranged in a region extending alongthe first casing element from the first end and having a pivot axis at amid portion of the first casing element; whereby upon release of one ofthe rotating aerofoil elements, the cantilever bends into the voidprovided between the first and second casing elements, which causes theat least one pocket to deform and to thereby absorb at least some of theimpact energy resulting from the released rotating aerofoil element. 2.A turbomachine casing assembly as claimed in claim 1, wherein the oreach pocket is formed from a sheet material selected from the groupcomprising thermoplastics, carbon fibre reinforced plastic, glass fibrereinforced plastic, aramid fibre reinforced plastic, boron fibrereinforced plastic and composites thereof.
 3. A turbomachine casingassembly as claimed in claim 1, wherein the or each pocket comprises askin formed from a rigid material and the or each pocket contains afluid.
 4. A turbomachine casing assembly as claimed in claim 1, whereinthe or each pocket is formed from a foam material.
 5. A turbomachinecasing assembly as claimed in claim 1, wherein each of the one or morerotating aerofoil elements comprises a first cutting portion on a radialend thereof.
 6. A turbomachine casing assembly as claimed in claim 1,wherein a second cutting portion is disposed at the first end of thefirst casing element.
 7. A turbomachine casing assembly as claimed inclaim 6, wherein a force transfer feature is disposed within the pocketand extending between the first casing element and the second cuttingportion.
 8. A turbomachine casing assembly as claimed in claim 1,wherein the or each pocket contains a liquid.
 9. A turbomachine casingassembly as claimed in claim 8, wherein the liquid is a contact adhesivewhich catalyses when in contact with air.
 10. A turbomachine casingassembly as claimed in claim 1, wherein the radially proximal face ofthe first casing element is contiguous with the radially proximal faceof the second casing element to form an uninterrupted surface.
 11. A gasturbine engine fan casing comprising the turbomachine casing assembly asclaimed in claim
 1. 12. A gas turbine comprising a turbomachine casingassembly as claimed in claim 1.